Pebble gas lift



United States Patent 2,724,618 PEBBLE GAS LlFT Louis C. Bearer, Bartlesville, Okla, assiguor to, Phillips Petroleum Company, a corporation of Delaware;

Application January 2, 1952, Serial No. 264,609 21 Claims. (Cl. 302-17) This invention relates to elevating solid. particles by a gas lift in a pebble heater type system. In one of its aspects this invention relates to a method and apparatus for maintaining continuous flow ina gas lift in a pebble heater system. In a preferred embodiment this invention relates to apparatus for automaticallyrelieving a blocked lift line in a gas lift ina pebble heater system.

Pebble heater type apparatus is finding increasing com mercial acceptance for effecting chemical reaction continuously at temperatures in the range of 1000 to 3500 F. The apparatus and process has many advantages when applied to hydrocarbon conversion reactions, such as thermal and catalytic cracking, reforming, dehydrogenation, dehydrocyclization or aromatization and the like. Some factors which favor the utilization. of pebble heaters are the extremely sharp heating rates possible with this type of apparatus, and the avoidance of contamination of the reaction product with the combustion gases.

The circulation of pebbles .in this system is effected, in part, by gravitational descent as a relatively compact mass of pebbles from an upper pebble heating zone where the pebbles are heated to a tempereature in the range of 1000 to 3500 F. by contact with hot gases, usually combustion gases from a burner, down through a conduit or throat into a conversion or heat transfer zone, where they are, directly and usually countercurrently contacted with a stream of hydrocarbon material. Such hydrocarbon material is heated by use of these said pebbles to a desired conversion temperature and subjected to the catalytic effect of :such pebbles, if any. From the lower part of said conversion zone pebbles which have given up a portion of their heat to the material undergoing conversion, pass through a duct and a pebble feeder into the lower portion of an elevation zone which usually consists of a mechanical elevator or a gas lift by Which means the pebbles :are elevated into a pebble settling zone and hopper from whence they return to the pebble heating zone, thus completing the cycle.

Preferably the contact material 0.1. pebbles employed is in the form of small, substantially spherical particles. Their size, whether spherical or other regular. or irregular shape, is sulficient that excessive pressure drop will not result when beds .of substantial .depths are employed in the heating and conversion zones. It is a further requirement, when a gas lift is employed rather than a mechanical elevator, that the solid particles be sufiiciently small to facilitate their transportation by said gas lift in that part of the circuit through which they flow between the conversion and the heating zones. Usually pebbles in the size range of Vs inch to 4 inch in diameter, preferably inch to. 3 2 inch diameter are employed. Pebbles made of alumina, beryllia, magn ia. hori zir nia, mullit periclase and other m terials, preferably refractory materials may be employed.

The presence in the system of substantial quantities of excessively fine particles of a powdery or dusty nature should be avoided so that they will not. form clinkers, or excessively fill the voids between the larger-pebbles and give an excessive pressure drop for the reactants and combustion gases, passing through the zones. For these reasons, the pebbles charged to the system are preferably of substantially uniform or well-graded size and any excessive. quantity of fines produced by attrition of the larger pebbles within the system is preferably removed therefromand replaced by larger pebbles. To avoid excessive attrition, the pebbles should have good structural strength to withstand extremely high temperature and shock conditions of temperaturechange, impact and abrasion to which they are subjected in the system. The use of substantially spherical pebbles will greatly assist in avoiding excessive attrition.

A special feature of my invention resides in the man ner and means whereby continuous circulation of the solid particles of catalyst or contact material is. maintained through the system. In most of the pebble heaters in commercial operation, the lifting or elevating means is comprised of a mechanical elevator which takes pebbles from the feeder below the reactor and elevates them to the top of the heating chamber. Some of the reactions which are carried out in pebble heater apparatus take place at exceedingly high temperatures and the temperature of the cooled pebbles is still above the effective operating range of mechanical elevators. In such instances, it is desirable and necessary to employ a gas lift. In elevating pebbles. by means of a gas lift the pebbles leaving the lower part of the reactor pass through a duct and pebble feeder into the lower portion of the lift conduit through which is passed a gas at a flow rate sufficient to fluidize and elevate said pebbles into a pebble settling zone and hopper. From there they complete the cycle back to the pebble heating zone. In order to avoid thermally shocking the pebbles during their concurrent flow upward to the settling chamber the temperature of the lifting fluid is. regulated to approximate that of the pebbles being lifted. In designing a gas lift for pebble heaters, many factors are taken into consideration such as the average density, size and shape of the pebble; the density of the lifting fluid; the terminal velocity of the gas necessary to freely suspend the pebbles; the total pressure drop in the lift including pressure drops due to gas friction pipe; and the diameter of the lift pipe. In a Well-designed gas lift there will be no blocking of the system due to a bridging in the pipe of the pebbles. However, in the normal operation of a gas lift, it has been found that if the flowing pebble density exceeds five pounds per cubic foot, the lift line will become blocked. Now, the flowing pebble density may approach and exceed this value due to several reasons such as, an inadvertent surge of pebbles through the pebble feeder or severe fluctuation in the rate of flow of the lifting gas. Usually when a blockage occurs in the lift the lift must be Shut down and the pebbles drained therefrom in some way or other. The present invention is directed to overcoming blockages of pebbles in the lift line of the pebble heater.

It is an object of this invention to provide. an improved method and apparatus for elevating solid par ticles by a gas lift in pebble heater type system.

It is another object of this invention to provide a method and apparatus for maintaining continuous flow in a gas lift in a pebble heater system.

A further objectof this invention is to provide apparatus for automatically relieving a blocked lift line in a gas lift in a pebble heater system.

Otherobjects and advantages will be apparent to those skilled in the art from the accompanying disclosure and discussion.

The accompanying drawing which is an elevation view portrays diagrammatically one embodiment of my invention.

Referring now tothe drawing in detail, substantially spherical pebbles A: inch to 4 inch, preferably inch to V2 inch in diameter, are gravitated as a relatively compact mass downwardly from an upper pebble heating zone 10, where they are heated to a temperature in the range of 1000 to 3500F. by contact with hot combustion gases from burner 11, through pebble duct 12 and into con version zone 13. The temperature to which the pebbles are heated in pebble heating zone can be controlled by any one of a number of conventional control devices, or in any one of a number of conventional ways. Combustion air for burner 11 is supplied from a blower, not shown. In conversion zone 13 hot pebbles are directly and countercurrently contacted with a hydrocarbon material undergoing conversion which enters that zone through line 14 at correlated flow rates so that said hydrocarbon material is heated by heat contained in said pebbles to a desired conversion temperature and for a desired time, and also receives the benefit of any catalytic eifect which may be exercised by the pebbles. Leakage of combustion gases from pebble heating zone 10 into conversion zone 13 or effluent gases from zone 13 into heating zone 10 caused by small differences in pressure between the two zones can be alleviated by the injection through line 15 into pebble duct 12 of an inert gas such as steam. The said pressure differences may be kept small, thus cutting down on the amount of seal steam required, by controlling the rate of flow of reaction efiluent leaving conversion zone 13 through line 16.

Pebbles which have given up part of their heat to the material undergoing conversion, leave conversion zone 13 through pebble duct and pebble feeder 21 passing into a pebble lift conduit 22 usually at a temperature considerably less than the conversion temperature. As a preferred method of operating, and in order to avoid thermally shocking the pebbles entering conduit 22, the lifting gas supplied by blower 23 is heated to the approximate temperature of the pebbles entering conduit 22 by means of burner 24. This step may be made automatic by employing a differential temperature controller connected to compare the temperature of the gas leaving the burner with that of pebbles going through conduit 20 and, in response to a predetermined temperature differential, control the valve in the primary air supply to the burner. The normal way of operating the burner is to have the primary air aspirate the fuel. The hot lift gas is passed upwardly through conduit 22 at a velocity sufficient to maintain the desired pebble circulation rate, thus elevating pebbles to the main pebble drop-out 25 and then through conduit 26 back to pebble heating zone 10, thereby completing the cycle. The lift gas need not be air if a source of gas at a suitable pressure is available.

The conditions at which a gas lift will be operated in a pebble heater, will of course depend primarily upon the pebble circulation rate which it is desired to maintain. For example, if it were desired to circulate 60,000 pounds per hour of pebbles at approximately 1000 F. through a lift pipe 10 inches in diameter and 90 feet high, it would require 2190 S. C. F. M. air at 1000 F. and a delivery pressure of 2.16 p. s. i. g. to accomplish the job. The calculated pressure drop in such a system would be 1.66 p. s. i. g. Thus, under such conditions it would be possible to maintain a back pressure of approximately 0.5 p. s. i. g. in the main pebble drop-out 25, provided that pebble conduit 26 were maintained pebble full so as to avoid excessive loss of the pressure. Assuming then 4 that the gas lift were to be operated under the conditions hereinbefore described, the back pressure on the main pebble drop-out 25 would vary in normal operation because of several factors, such as fluctuation in the rate of discharge from pebble feeder 21. If more pebbles than were desired were passed into the lift the back pressure in drop-out 25 would drop due to the increased pressure drop across the lift line and conversely, if less pebbles entered the lift the back pressure would increase due to a decreased pressure drop across the lift line. This invention is directed to anticipating and alleviating a blocked lift without shutting down the lift entirely. Therefore, I am concerned only with those variables which tend to cause an increase in the pebble flow rate and thus cause a drop inthe back pressure maintained in the pebble drop-out 25. In the present operation it is reasonable for the back pressure in pebble drop-out 25 to vary, that is drop, from 0.5 to 0.2 p. s. i. g. without causing a block in the line. However, any drop greater than that would indicate that a block has occurred, or will occur unless corrective measures are taken. It has been my experience that when a block does occur in the lift line, usually it is located at a point from 3 to 10 feet above the base of the air lift line depending on the particular arrangement employed especially that of the location of the inlet of the pebbles into the lift. According to this embodiment of my invention when operating at the conditions hereinbefore described if the back pressure in main pebble dropout 25 drops more than .3 of a pound thus indicating a blocked lift line, or an anticipated blocked lift line, a detected by pressure control device 30, sol noid operated valve 31 in line 32 supplying high pressure instrument air opens normally closed air-operated motor valve 33. When this occurs the lift gas then passes through by-pass conduit 34, the pebbles from the feeder fall downwardly in the main lift from their point of entry to the entrance to conduit 34 and then pass upwardly in by-pass conduit 34 and fall out in auxiliary drop-out chamber 35. At the same time the agglomerated pebbles fall freely to the bottom of the lift and are passed upwardly through bypass conduit 34 and into auxiliary drop-out 35. All this usually takes place in a matter of 15 to 20 seconds, or shorter. When the normal back pressure in main drop-out 25 has been restored, pressure control device 30 causes solenoid valve 31 to close thus closing air operated motor valve 33. Motor valve 33 is preferably an air-operated valve with a bleed orifice to eliminate or minimize the possibility that the said valve would close before the pebble blockage has been relieved. If a different type of valve is employed rather than an air-operated motor valve, it may be desirable to incorporate a brief time delay device in the circuit controlling the operation of this valve. Bypass conduit 34 is connected into main pebble lift conduit 22, /2 to 2 feet above the base of the air lift, and about a similar distance below the point of entry of the pebbles from conduit 20, depending on the arrangement used. Valve 37 in line 36 connecting the bottom of auxiliary drop-out 35 with main pebble lift conduit 22 is normally kept closed. The pebbles which accumulate in the auxiliary pebble drop-out may be returned periodically to the main conduit at such a rate as not to interfere with the operation of the lift.

It will be appreciated that different methods and apparatus can be employed to determine the existence of at least an impending block in the lift conduit. By impending block I mean that point in the deviation of a selected variable beyond which a block is inevitable unless corrective measures are taken. Another modification of my invention involves metering the rate of flow of gaseous efiluent vented from main pebble drop-out chamber 25 which rate should remain substantially constant in normal operation, and in response to a predetermined drop in said flow rate sufficient inv magnitude to indicate at least an impending block automatically causing valve 33 to be opened as hereinbefore described. When the normal rate of flow of gaseous efiluent vented from drop-out 25 is restored valve 33 would be closed. Of course, in this modification it is necessary to maintain conduit 26 pebble full andto assume that the loss of lift gas therethrough remains substantially constant for any given set of operating conditions. p

Another embodiment of my invention involves the use of a pressure controller connected to detect a buildup of pres sure in the bottom of the lift to a predetermined value of magnitude so as to indicateat least an impending block in conduit 22, and in response thereto automatically cause valve 33 to be opened, and to be closed when the said pressure falls to normal. It is preferred in this embodiment to employ a positivedisplacement type blower. However, a centrifugal typeblower can be employed in this embodiment or any of the others mentioned.

Stillanother modification of my invention involves the use of a differential pressure controller connected to measure the differential pressure across conduit 22 between the point of pebble entry and drop-out 25'. For any given pebble flow rate the differential pressure between the two said points will remain substantially constant. However, when the said differential pressure increases a predetermined amount the magnitude of which is sufficient to indicate at least an. impendingblock in conduit 22, the differential pressure controller will cause valve 33 to be opened, and to be closed when the differential pressure falls to normal or below. This will happen very quickly, but it doesnt take long to clear a block in the line. An advantage to operating my invention according to these last two embodiments isthat pebble conduit 26 need not be kept pebble full and the lift gas can be vented to the atmosphere without metering of any kind.

It will be appreciated that the degree of variation in the particular variable selected for observation which will indicate at least an impending block in main lift conduit 22 will be dependent upon many factors, the most important being the pebble circulation rate to be maintained.

While this invention is described and exemplified in terms of its preferred embodiment, it will be appreciated that modifications may be made without departing from the invention.

1 claim:

1. A method for preventing a block in a main gas lift conduit in a pebble heater system which comprises, detecting at least an impending block in the main lift conduit by detecting a predetermined variation in a preselected variable and in response to said variation Withdrawing pebbles from said main lift conduit into a by-pass conduit, and passing all ofsaid pebbles and lift gas through said main lift conduit when the value of said preselected variable returns to normal.

2. A method for preventing a block in a main gas lift conduit in a pebble heater system, which comprises, maintaining a normally fixed back pressure on the main pebble drop-out zone and in response to a drop in said back pressure the magnitude of which indicates at least an impending block in the main lift line, withdrawing pebbles and lift gas from said main lift conduit into a by-pass conduit, removing said withdrawn pebbles from said by-pass conduit in an auxiliary drop-out zone, and passing all of said pebbles and lift gas through said main lift conduit when the normally fixed back pressure in the main pebble drop-out zone is restored.

3. A method for preventing a block in a main gas lift conduit in a pebble heater system, which comprises, maintaining a normally fixed differential pressure in the main lift conduit between the main pebble drop-out zone and the point of pebble entry, and in response to a predetermined increase in said differential pressure the magnitude of which indicates at least an impending block in the main lift conduit, withdrawing pebbles and lift gas from said main lift conduit into a by-passconduit, removing said withdrawn pebbles from said bypass conduit in an auxiliary drop-out zone, and passing all of said pebbles and 6 liftgas; through said main lift conduit, when the value, of the said differential pressure returns to normal.

4. A method for preventing a block in a main gas lift conduit in a pebble heater system, which comprises, maintaining a normally fixed rate of flow of gaseous effluent from the main pebble drop-out zone and in response to a drop in said rate of flow the magnitude of which indicates at least an impending block in the main, pebble lift conduit, withdrawing pebbles and lift gas from said main lift conduit into a by-pass conduit, removing said withdrawn pebbles from said by-pass conduit in an auxiliary drop-out zone, and passing all of said pebbles and lift gas through said main lift conduit when the value of said rate of flow of gaseous efiluent from said main pebble drop-out zone returns to normal.

5., A method for preventing a block in a main gas lift conduit in a pebble heater system, which comprises, maintaining a normally fixed operating pressure in the bottom of the main lift conduit, and in response to a predetermined build-up in said operating pressure of magnitude suflicient to indicate at least an impending block in the main lift conduit, withdrawing pebbles and lift gas from said main lift conduit into a by-pass conduit, removing said withdrawn pebbles from said bypass conduit in an auxiliary drop-out zone, and passing all of said pebbles and lift gas through said mainlift conduit when the value of said operating pressure returns to normal.

6. A gas lift for a pebble heater which comprises, in combination, a source of lift gas; a main lift conduit system connected at the bottom to said source; amain pebble drop-out chamber connected at the top of said main conduit system; a bypass conduit system connected into said main lift conduit at the bottom below the the point of pebble entry and at the top into said main pebble drop-out chamber; an auxiliary pebble drop-out chamber connected in said by-pass conduit system; a valve, normally closed, connected in said bypass conduit system; means for automatically opening and closing said valve; means for detecting at least an impending block in the main lift conduit by detecting a predetermined variation in a preselected variable, said second-mentioned means automatically operating said first-mentioned means to open said valve when the said predetermined variation is detected and closing said valve when the value of said preselected variable returns to normal.

7. A gas lift for a pebble heater which comprises, in combination, a blower; a main lift conduit system connected at the bottom to said blower; a burner connected in said main lift conduit system so as to preheat the lift gas; a main pebble drop-out chamber connected at the top of said main conduit system; a by-pass conduit system connected into said main lift conduit at the bottom onehalf to two feet below the point of pebble entry and at the top into said main pebble drop-out chamber; an auxiliary pebble drop-out chamber connected in said by-pass conduit system; a motor valve, normally closed, connected in said by-pass conduit system; a source of energy to operate said last-mentioned motor valve; means connected to said source of energy for automatically opening and closing said motor valve; and a pressure control device connected to detect a normally fixed back pressure in said main pebble drop-out chamber, and in response to a predetermined drop in said back pressure, the magnitude of which is suflicient to indicate at least an impending block in the main lift conduit system, to automatically operate said means to open said motor valve and to automatically close said motor valve when the value of the normally fixed back pressure in said main pebble drop-out chamber returns to normal.

8. A gas lift for a pebble heater which comprises, in combination, a positive displacement blower; a main lift conduit system connected at the bottom to said blower; a burner connected in said main lift conduit system so as to preheat the lift gas; a main pebble drop-out chamber connected at the top of said main conduit system; a bypass conduit system connected into said main lift conduit at the bottom one-half to two feet below the point of pebble entryand at the top into said main pebble dropout chamber; an auxiliary pebble drop-out chamber connected in said by-pass conduit system; an air-operated motor valve, normally closed, connected in said by-pass conduit system; a source of air to operate said motor valve; solenoid-operated valve means connected to said source of. air for automatically opening and closing said airoperated motor valve; and a pressure controller connected to detect a normally fixed operating pressure at the bottom of said main lift conduit and in response to a buildup in said operating pressure, the magnitude of which indicates at least an impending block in said main pebble lift conduit, to automatically operate said solenoid-operated valve means to open said air-operated motor valve, and to automatically close said air-operated motor valve when the value of said operating pressure returns to normal.

9. A gas lift for a pebble heater which comprises, in combination, a centrifugal blower; a main lift conduit system connected at the bottom to said blower; a burner connected in said main lift conduit system so as to preheat the lifting fluid; a main pebble drop-out chamber connected at the top of said main conduit system; a by-pass conduit system connected into said main lift conduit at the bottom one-half to two feet below the point of pebble entry and at the top into said main pebble drop-out chamber; an auxiliary pebble drop-out chamber connected in said by-pass conduit system; an air operated motor valve, normally closed, connected in said by-pass conduit system; a source of air to operate said motor valve; a solenoidoperated valve means for automatically opening and closing said air-operated motor valve; and a rate of flow controller connected to detect a normally fixed rate of flow of gaseous effluent from said main pebble drop-out chamber, and in response to a drop in said rate of flow, the magnitude of which is sufficient to indicate at least an impending block in said main pebble lift conduit, to automatically operate said solenoid-operated valve means to open said air-operated motor valve, and to automatically close said air-operated motor valve when the value of the rate of flow of gaseous eflluent from said main pebble dropout chamber returns to normal.

10. A gas lift for. a pebble heater which comprises, in combination, a centrifugal blower; a main lift conduit system connected at the bottom to said blower; a burner connected in said main lift conduit system so as to preheat the lift gas; a main pebble drop-out chamber connected at the top of said main conduit system; a by-pass conduit system connected into said main lift conduit at the bottom one-half to two feet below the point of pebble entry and at the top into said main pebble drop-out chamber; an auxiliary pebble drop-out chamber connected in said bypass conduit system; an air-operated motor valve normally closed connected in said by-pass conduit system; a source of air to operate said motor valve; a solenoid-operated valve means connected to said source of air to automatically open and close said air-operated motor valve; and a differential pressure controller connected to detect a normally fixed differential pressure across said main lift conduit between the main pebble drop-out chamber and the point of pebble entry therein, and in response to an increase in said differential pressure of magnitude sufiicient to indicate at least an impending block in said main pebble lift conduit, to automatically operate said solenoidoperated valve means to open said air-operated motor valve, and to automatically close said air-operated motor valve when the value of said differential pressure returns to normal.

11. In a method for elevating granular material from a lower zone to an upper zone through a main elevating conduit by a lift gas, the improvement of preventing a block in said main elevating conduit which comprises, passing said granular material in the lower portion of said main elevating conduit through a granular material entry 8 point, admitting lift gas into said main elevating conduit below said granular material entry point, detecting at least an impending block in said main elevating conduit by detecting a predetermined variation in a preselected variable, and withdrawing a portion of said granular material from said main elevating conduit in response to said predetermined variation in said preselected variable.

12. In a method for elevating granular material from a lower zone to an upper zone through a main elevating conduit by a lift gas, the improvement of preventing a block in said main elevating'conduit which comprises, passing said granular material into the lower portion of said main elevating conduit through a granular material entry point, admitting lift gas into the bottomof said main elevating conduit, detecting at least an impending block in said main elevating conduit by detecting a predetermined variation in a preselected variable, withdrawing a portion of the granular material and lift gas from said main elevating conduit into a by-passconduit in response to said predetermined variation in said preselected variable, and passing all of said granular material and lift gas'through said main elevating conduit when the value of said preselected variable returns to normal.

13. A method in accordancewith claim 12 wherein said predetermined variation in said preselected variable is a drop in a normally-fixed back pressure on said upper zone of a magnitude which indicates at least an impending block in said main elevating conduit.

14. A method in accordance with claim 12 wherein said predetermined variation in said preselected variable is an increase in a normally-fixed differential pressure in said main elevating conduit between said upper zone and said granular material entry point of a magnitude which indicates at least an impending block in said main elevating conduit.

15. A method in accordance with claim 12 wherein said predetermined variation in said preselected variable is a drop in a normally-fixed rate of flow of gaseous effluent from said upper zone of a magnitude which indicates at least an impending block in said main elevating conduit.

16. A method in accordance with claim 12 wherein said predetermined variation in said preselected variable is a build-up in a normally-fixed operating pressure in the bottom of said main elevating conduit of a magnitude which indicates at least an impending blockin said main elevating conduit.

17. In an apparatus for elevating granular material from a lower vessel to an upper vessel through a main elevating conduit which is in communication with both vessels and which comprises a source of lift gas connected into the bottom of said main elevating conduit and a granular material inlet connected into the lower portion of said elevating conduit, the improvement which comprises a by-pass conduit connected into said main elevating conduit below said granular material inlet, a normally-closed valve in said by-pass conduit, means for opening and closing said valve, means for detecting at least an impending block in said main elevating conduit by detecting a predetermined variation in a preselected variable, said second-mentioned means operating said first-mentioned means to open said valve when said predetermined variation is detected and to close said valve when the value of said preselected variable returns to normal.

18. An apparatus in accordance with claim 17 wherein said second-mentioned means is a pressure control device connected to detect a normally-fixed back pressure in said upper vessel and which, in response to a predetermined drop in said back pressure the magnitude of which is sufficient to indicate at least an impending block in said main elevating conduit, operates said first-mentioned means to open said valve in said by-pass conduit and to close said valve in said by-pass conduit when the value of the normally-fixed back pressure in said upper vessel returns to normal.

19. An apparatus in accordance with claim 17 wherein said second mentioned means is a pressure controller connected to detect a normally-fixed operating pressure at the bottom of said main elevating conduit and which, in response to a build-up in said operating pressure of a magnitude which indicates at least an impending block in said main elevating conduit, operates said firstmentioned means to open said valve in said by-pass conduit and to close said valve in said by-pass conduit when the value of said operating pressure returns to normal.

20. An apparatus in accordance with claim 17 wherein said second-mentioned means is a rate of flow controller connected to detect a normally-fixed rate of flow of gaseous efiluent from said upper vessel, and which, in response to a drop in said rate of flow of a magnitude which is sufiicient to indicate at least an impending block in said main elevating conduit, operates said firstmentioned means to open said valve in said by-pass conduit and to close said valve in said by-passconduit when the value of the rate of flow of gaseous eflluent from said upper vessel returns to normal.

21. An apparatus in accordance with claim 17 wherein said second-mentioned means is a diflerential pressure controller connected to detect a normally-fixed dififerential pressure across said main elevating conduit between said upper vessel and said granular material inlet, and which, in response to an increase in said difierential pressure of a magnitude sulficient to indicate at least an impending block in said main elevating conduit, operates said first-mentioned means to open said valve in said by-pass conduit and to close said valve in said by-pass conduit when the value of said differential pressure returns to normal.

References Cited in the file of this patent UNITED STATES PATENTS 620,854 Schuman Mar. 7, 1899 1,264,601 Bernert Apr. 30, 1918 2,554,583 McFall May 29, 1951 2,561,409 Ardern July 24, 1951 2,623,793 Hill Dec. 30, 1952 

1. A METHOD FOR PREVENTING A BLOCK IN A MAIN GAS LIFT CONDUIT IN A PEBBLE HEATER SYSTEM WHICH COMPRISES, DETECTING AT LEAST AN IMPENDING BLOCK IN THE MAIN LIFT CONDUIT BY DETECTING A PREDETERMINED VARIATION IN A PRESELECTED VARIABLE AND IN RESPONSE TO SAID VARIATION WITHDRAWING PEBBLES FROM SAID MAIN LIFT CONDUIT INTO A BY-PASS CONDUIT, AND PASSING ALL OF SAID PEBBLES AND LIFT GAS THROUGH SAID MAIN LIFT CONDUIT WHEN THE VALVE OF SAID PRESELECTED VARIABLE RETURNS TO NORMAL. 